Epigenetic Mechanism Helps Strengthen Memories

In biology, memories have always had an enigmatic quality that has often eluded researchers studying their formation and retention. Yet now, investigators at the University of Alabama at Birmingham (UAB) have just discovered a novel mechanism that links epigenetic changes in the hippocampus of the rat brain that controls downstream regulation of translation in brain neurons during fear memory reconsolidation. Understanding how memories form and are retrieved has applications to psychiatric, neurological, and neurodegenerative disorders, and may be helpful to attenuate maladaptive memories in psychiatric disorders.

Previously, scientists uncovered two broad mechanisms for memory reconsolidation, which is the retrieval and strengthening of recent memory. The first is that during memory reconsolidation, changes in translational control—the process of forming new proteins from activated genes—occur in areas of the brain related to memory formation. The second broad finding is that epigenetic mechanism—various molecular modifications are known to alter the activity of genes without changing their DNA sequence—are also somehow actively involved during memory reconsolidation or strengthening.

In this new study, UAB researchers found that the memory reconsolidation events were acting through a gene called Pten (phosphatase and tensin homolog). The downstream target affected by changes in PTEN enzyme levels is the AKT-mTOR pathway, one of the main translation control pathways involved in memory reconsolidation. PTEN was already known to be a potent inhibitor of AKT-mTOR but was not previously linked to epigenetic control of memory.

“These findings could be critical in the treatment of memory disorders, such as post-traumatic stress disorder,” explains senior study investigator Farah Lubin, Ph.D., associate professor in the UAB department of neurobiology. “PTSD is thought to be caused by the lack of extinction of fear memory. Altering this memory during the reconsolidation process could help in re-associating the memory with a less traumatic context.”

The UAB team found that retrieval of contextual fear memory in rats briefly increased levels of the enzyme EZH2, an enzyme known to add methyl groups to histones. Histones are proteins that help package and order DNA in the chromosome, and they also play a role in epigenetic gene regulation. Along with the increased EZH2, the researchers found increased methylation of histone H3, specifically the addition of three methyl groups to the lysine 27 amino acid of histone H3. That trimethylation of the histone by EZH2, known as H3K27me3, correlated with decreased levels of PTEN enzyme.

“We found that in male rats retrieval of a contextual fear memory transiently increased Enhancer of Zeste Homolog 2 (EZH2) levels along with increased histone H3 lysine 27 trimethylation (H3K27me3) levels, which correlated with decreased levels PTEN, a potent inhibitor of AKT–mTOR-dependent signaling in the hippocampus,” the authors wrote. “Further experiments found increased H3K27me3 levels and DNA methylation across the Pten promoter and coding regions, indicating transcriptional silencing of the Pten gene.”

Interestingly, examination of the DNA encoding for the Pten gene showed increased levels of H3K27me3 bound to the DNA, as well as DNA methylation, across the promoter and coding regions of the Pten gene. DNA methylation is another form of epigenetic control, and both the histone epigenetic change and the DNA epigenetic change indicated transcriptional silencing of the Pten gene.

The scientists subsequently used small interfering RNA, or siRNA, to knock down genes. Through knockdown experiments, they showed that levels of H3K27me3 and PTEN appear to control the AKT-mTOR pathway directly.

Knockdown of the gene for the H3K27me3 methyltransferase enzyme, Ezh2, in the hippocampus, which is the memory consolidating region of the brain, prevented decreases of PTEN and activation of AKT-mTOR during memory reconsolidation. But when the Ezh2 and Pten genes were knocked down at the same time, the AKT-mTOR pathway was activated.

“We found that H3K27me3 regulated Pten repression necessary for mTOR phosphorylation during memory reconsolidation,” Lubin concluded. “As a result, we have identified a novel epigenetic pathway critical for regulation of translational control mechanisms during memory reconsolidation.”